Induction heat treatment of an annular workpiece

ABSTRACT

Apparatus and method are provided for inductively heat treating a circular surface of annular workpieces where at least one inductor pair is used to perform a scan induction heat treatment of the circular surface. Controlled movement of the inductors and application of quenchant is provided particularly at the initial and final heat treatment locations on the circular surface to enhance metallurgical uniformity of the annular workpiece at these locations. In combination with controlled movement of the inductors, a simultaneous power-frequency control scheme can be applied to the inductors during the heat treatment process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/323,428 filed Apr. 13, 2010, hereby incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to induction heat treatment ofannular workpieces, and in particular to when at least one pair ofinductors are utilized in a scan induction heat treatment process of oneor more surfaces of an annular workpiece.

BACKGROUND OF THE INVENTION

Electric induction heating can be used to heat electrically conductivematerials (for example, cast irons and steels) to temperatures in theaustenitic range. The heated material is then quenched to temperatureswhere low transformation products, such as martensite and/or bainite areformed. There are two basic approaches to inductively heating a largeannular, or ring-shaped workpiece, namely a single-shot (static) processor a scan process.

In a static induction heating process the region of the workpiece thatis required to be heat treated can be surrounded by a single-turn ormulti-turn induction coil. For example to metallurgically harden aregion on the inside diameter 90 a of annular workpiece 90 (FIG. 1( a)),an induction coil can be positioned inside of the formed annulus, andalternating current (AC) is supplied to the induction coil to establisha magnetic field around the coil that provides an electromagnetic fluxcoupling with the inside diameter region of the workpiece for thedesired heat treatment. If heat treatment of a region (shown as shadedregion 90 c in FIG. 1( c)) on the outside diameter 90 a′ of workpiece90, then induction coil 100 can be positioned outside of the formedannulus as shown in FIG. 1( b) and FIG. 1( c). Induction coil 100 isconnected to an alternating current power source 102. In thisarrangement, induction coil 100 encircles the outer diameter ofworkpiece 90. The workpiece can be optionally rotated (for example aboutworkpiece central axis A) during the heat treatment process to ensure aneven distribution of induced energy around the workpiece's perimeterover the entire heating cycle. Rotation rates are selected to suitprocess requirements.

When utilizing an encircling induction coil 100 as shown in FIG. 1( a)and FIG. 1( b), the following process parameters play a dominant role inobtaining the required hardness depth, δ, and pattern: frequency of thesupplied alternating current; magnitude of the supplied induction power;quenching parameters (such as temperature of the quenchant; quenchantrate of flow (flux density); pressure and concentration of quenchant,for example with aqueous polymer quenchant); and cycle process time.Cycle process time includes: induction heating time; soaking time (ifsoaking is used); and quenching time. There are two commonly appliedmethods of quenching in a single-shot heating process of a large annularworkpiece. According to one technique as illustrated in FIG. 1( d), uponcompletion of the induction heating stage, the heated workpiece ispositioned within a separate concentric spray quench block (or ring) 104that is positioned below the inductor 100 and spray-quenched in-place bymoving workpiece 90 downwards as shown in FIG. 1( d). Upon sufficientquenching, a surface hardness layer 90 c′ will be formed on the surfaceof the workpiece. In an alternative quenching method as illustrated inFIG. 1( e), the heated annular workpiece 90 is submerged in a quenchtank 92 filled with quenchant 94 and quenching takes place inside of thequench tank while the quenchant is usually agitated by suitable means.

One of the main drawbacks of a single-shot heat treatment is thenecessity of supplying the induction coil (inductor) with a substantialamount of power since the simultaneous heating method requires amagnitude of power sufficient to raise the temperature of the entiresurface of the ring to the required level at required depth. Thereforecostly high power induction heating sources are required.

In a scan induction process, an appreciably smaller inductor than thatused in the single-shot process, such as short inductor 101 moves in acircular path (concentric with the center of the workpiece) around theouter perimeter of annular workpiece 90 as shown in FIG. 2( a). Singleinductor 101 is shown multiple times in FIG. 2( a) and FIG. 2( b) toindicate the directed circular travel path of the inductor, namely fromstart position A1, followed by sequential (clockwise CW) subsequentquadrant positions B1, C1 and D1. While moving around the workpiece themagnetic flux field established by alternating current flow in inductor101 couples to a required penetration depth of the workpiece asdiagrammatically shown by shaded regions. Single spray quench apparatus105 moves with (tracks) inductor 101 around the workpiece and islikewise shown multiple times in the figures. Spray quench apparatus 105may be of suitable form known in the art such as a quench block or jet,and may also be an integral assembly with the inductor. This scaninduction process requires significantly less power than the single-shotprocess since only a small sector of the workpiece is instantaneouslyflux coupled and inductively heated as inductor 101 moves around theannular workpiece. A disadvantage of this method is the presence of a“soft” zone 90 d in the metallurgically hardened (shaded) penetrationdepth 90 c′ as shown in FIG. 2( b) where the workpiece will not beproperly heat treated. The soft zone in this example is a function ofthe length of the coil 101 and its scan speed and is generally in therange of 1 to 9 cm in arc length as shown in FIG. 2( b). The term “softzone” is used to describe a region where the desired metallurgical heattreatment achieved in the penetration depth elsewhere around the outerperimeter is not achieved. Soft zone 90 d is inevitably created due tothe tempered region adjoining the final ring section to be heated.

To prevent soft zones while scan hardening without the requirement foran oversized power supply, as required with static one shot hardening,the prior art double inductor/quench apparatus arrangement shown in FIG.3 can be utilized. A pair of inductors 103 a and 103 b can be used witheach inductor in the pair performing induction hardening for one-half ofthe annular workpiece 90. In FIG. 3 each inductor surrounds the innerand outer perimeters of the workpiece so that penetration depths intothe inner and outer perimeters are heat treated. The arrangement shownin FIG. 3 is further described in “Induction Surface Hardening” by A. D.Demichev, pages 25-26, published by the Leningrad Division of PublishingHouse “Mashinostryeniye”, Saint Petersburg, RUSSIA, 1979. For simplicityin illustration and description FIG. 4( a) through FIG. 4( c) areprovided to describe a double inductor/quench apparatus arrangementwhere only a penetration depth from the outer perimeter of the workpieceis heat treated. Inductors 103 a (counterclockwise) and 103 b(clockwise) move in circular counter directions at a constant speedaround the outer perimeter of workpiece 90 from starting positions A1and A2 respectively as shown in FIG. 4( a) through intermediatepositions B1 and B2, respectively, as shown in FIG. 4( b), and then tofinish positions C1 and C2, as shown respectively, in FIG. 4( c). Thecounterclockwise arc and clockwise arc from position A1 to position C1and position A2 to position C2 respectively are less than 180 degreesdue to the physical space taken up by both inductors when they areadjacent (side-by-side) to each other at the start and finish positions.Each inductor is supplied the same magnitude of power from a suitablealternating current source through the less than complete semicircularmovement around the outer perimeter of the workpiece. As with the singleinductor process described above spray quench apparatus 105 a and 105 bmoves with (tracks) inductor 103 a and 103 b respectively, around theworkpiece until the inductors are adjacent to each other at the end ofthe heating process in positions C1 and C2 as shown in FIG. 4( c). Bothspray apparatus are de-energized at these positions and, simultaneously,an auxiliary spray apparatus 105 c automatically provides quenchant tothe final heat treated sector 90 e of the workpiece as shown in FIG. 4(c). The adjacent inductors in the final heating positions C1 and C2eliminate the presence of soft zones in the final heating positions.

One of shortcomings of the double inductor/spray apparatus process isthe difficulty in providing uniform heating, and as a result, a uniformhardness depth 90 c in the start and finish positions (A1, A2 and C1,C2). At the start of the heating process, the distance between inductors103 a and 103 b can not be immediately adjacent to each other since themagnetic fields established by current flow in each inductor couldinterfere with each other if supplied by independent power supplies,which can result in lower levels of induced heating.

Additionally after the heating process starts, both inductors 103 a and103 b have to travel sufficiently far from each other before quenchantcan be supplied from quench apparatus 105 a and 105 b to heated region90 e of the workpiece 90 as shown in the detail views of FIG. 5( a) andFIG. 5( b). If quenchant is supplied too soon (that is, when theinductors have not traveled sufficiently far apart from each other),quenchant can splash onto heating sectors located under the energizedinductors, which results in the formation of unacceptable hardeningstructures, such as an appearance of regions within the hardness patternhaving inappropriate phase transformations, soft spots, and alteredmicrostructures. Therefore, there is always a longer quench delay duringthe initial induction heating stage compared to the quench delay duringscanning.

Both inductors 103 a and 103 b must travel in opposite directionssufficiently far from each other to avoid quench splashing on the zonebeing heated as shown in FIG. 5( c) before quench spray 105 a′ couldbegin to be supplied from quench apparatus 105 a and 105 b. Typicallythis separation distance can be in the approximate range of 5 to 10 cm.During this unavoidable quench delay time period, there will be heatloss from the previously heated region between inductors 103 a and 103 bdue to a thermal conduction that leads to a heat flow from hightemperature regions of the ring towards its cooler regions resultingfrom a “cold sink effect.” Due to this effect, the previously heatedarea can cool down to temperatures below the level, and at a rate that,is too slow for obtaining a desired fully martensitic structure. Duringinevitable quench delay, besides the cold sink effect, cooling of theinitially heated areas take place due to surface heat losses fromthermal radiation and convection. Greater “hardness depth-to-ringthickness” ratios and slower scan speeds of the inductors negativelyaffect thermal conditions of the initially heated region that ispositioned between inductor pair 103 a and 103 b. A similar difficultyin achieving a desired temperature distribution and hardness profileoccurs in the final heating region (positions C1 and C2) of theworkpiece as shown in FIG. 4( c) for reasons related to quench delaysimilar to those described above for the start positions of theinductors.

One object of the present invention is to achieve a metallurgicallyuniform hardness layer in the region where the induction heating processbegins and ends in a two or more inductor/spray apparatus employing ascan heat treatment process for an annular workpiece.

BRIEF SUMMARY OF THE INVENTION

In one aspect the present invention is a method of, and apparatus for,scan induction heat treatment of an annular workpiece where at least twoinductors are simultaneously used. Controlled movement of the inductorsand application of quenchant is provided at the initial and finalheating locations of the two inductors to enhance metallurgicaluniformity of the annular workpiece at these locations. In combinationwith controlled movement of the inductors, a simultaneouspower-frequency control scheme can be applied to the inductors.

The above and other aspects of the invention are set forth in thisspecification and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings, as briefly summarized below, are provided forexemplary understanding of the invention, and do not limit the inventionas further set forth in this specification and the appended claims:

FIG. 1( a) is an isometric view of one example of an annular or ringworkpiece that can be metallurgically heat treated by the method andapparatus of the present invention.

FIG. 1( b) and FIG. 1( c) are diagrammatic top view and cross sectionalview (through line B-B in FIG. 1( b), respectively, of a typical priorart single-shot induction heat treatment process for an annularworkpiece.

FIG. 1( d) and FIG. 1( e) illustrate two typical prior art methods ofquenching the heated workpiece in FIG. 1( b) and FIG. 1( c).

FIG. 2( a) and FIG. 2( b) diagrammatically illustrate a prior art singleinductor and quench apparatus method of scan induction heating andquenching to a metallurgical hardening penetration depth around theouter perimeter of an annular workpiece.

FIG. 3 is a diagrammatic top view of one prior art scan inductionapparatus utilizing two inductors that metallurgically heat treat boththe inside and outside perimeters of an annular workpiece.

FIG. 4( a) through FIG. 4( c) diagrammatically illustrate a prior artscan induction process utilizing two inductors that metallurgically heattreat the outside perimeter of an annular workpiece.

FIG. 5( a) through FIG. 5( c) diagrammatically illustrate in detail theinitial heating stage process for the prior art scan induction processshown in FIG. 4( a) through FIG. 4( c).

FIG. 6( a) through FIG. 6( d) diagrammatically illustrate initialheating stage process steps of the present invention for a dual inductorand quench block scan induction heat treatment process for the outsideperimeter of an annular workpiece.

FIG. 6( e) diagrammatically illustrates one example of a steady stateinduction heat treatment process step between the initial heating stageand the final end of heat treatment process steps.

FIG. 7( a) through FIG. 7( e) diagrammatically illustrate twoalternative examples of the final heating stage process steps of thepresent invention for a dual inductor and quench block scan inductionheat treatment process for the outside perimeter of an annularworkpiece.

FIG. 8 diagrammatically illustrates one example of initial heating stageprocess steps of the present invention and two alternative examples of afinal end of heat treatment process steps of the present invention.

FIG. 9( a) through FIG. 9( e) graphically illustrate one example of aninduced power-frequency control scheme for application with the presentinvention.

FIG. 10( a) through FIG. 10( f) diagrammatically illustrate alternativeexamples of the final heating stage process steps of the presentinvention for a dual inductor and quench block scan induction heattreatment process for the outside perimeter of an annular workpiece.

FIG. 11 illustrates one example of an apparatus of the present inventionthat can be used to practice some of the examples of the inductionheating processes of the present invention.

FIG. 12 is a detail view of a pair of inductor assemblies used in theapparatus shown in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

The term annular (ring) workpiece is used to describe an annularcomponent, such as, but not limited to, a large roller or ball bearingrace. Such bearing races can be used, for example, in thrust bearings inwind turbines that are capable of producing electric power in themegawatt range. If the workpiece is a large bearing race, the surface,or surfaces that may be induction heat treated are the inner and outercircular races (90 a and 90 a′ respectively in FIG. 1( a)) and axialraces (90 b in FIG. 1( a); lower axial race not visible). The relativeterm “large” is used herein to describe an annular workpiecesufficiently large to be affected by the deficiencies described abovefor the prior art dual inductor scan induction heat treatment process;such an annular workpiece typically has an inside diameter ofapproximately one meter or larger.

An example of the induction metallurgical heat treatment process of thepresent invention is illustrated in FIG. 6( a) through FIG. 7( e)utilizing dual inductors 12 a and 12 b, with associated quench apparatus14 a and 14 b, respectively. Workpiece 90 may be, by way of example andnot limitation, a ring bearing race surface having an inner diameterexceeding 1 meter. Inductors 12 a and 12 b can typical be what is knownas “hairpin” inductors since they can be formed of bent copper tubing toconform to shape of the annular workpiece's surface to bemetallurgically hardened (also referred to as profiling). Magnetic fluxconcentrators can optionally be used to concentrate the magnetic fieldand improve heating efficiency. Not shown in these figures are suitablemounting structures for the inductors and quench apparatus that hold andmove the inductors and quench apparatus as describe herein. Alternating(AC) current is supplied to each of the inductors from one or moresuitable sources. The AC current may be controlled to vary in frequencyand power over a heating process; generally the AC current to bothinductors are in phase. Quench apparatus 14 a and 14 b are showndiagrammatically in a stacked configuration with their respectiveinductors 12 a and 12 b. Quenchant is supplied from a suitable source tothe quench apparatus and quenchant outlets in the quench apparatusdirect quenchant flow (spray) towards the surface of the annularworkpiece that was previously inductively heated by their respectiveinductors in the manner further described below. Further the quenchapparatus (or spray blocks) may be pivotly connected either to aninductor supporting structure or other supporting structure forcontrollable direction of spray impingement on a heated region of theworkpiece.

The initial step in the heat treatment process of the present inventionis an oscillatory heating step within oscillation zone OSC shown in FIG.6( a) and FIG. 6( b). In this initial step inductors 12 a and 12 b canbe located as close together side-by-side (represented by distance d₁ inFIG. 6( c)) as permitted by the physical limitations of a particulardesign of the inductors with starting position being anywhere withinoscillation zone OSC. As an order of magnitude, the side-by-sideseparation of the inductors is typically within the range of 1 to 5 cm,which can be a significant region of non-uniform heat treatment forlarge annular workpieces when installed as a component in a particularapplication. For convenience, when the center of zone OSC is located atdesignated position “0” (at three o'clock), the process startingposition for the inductors may be at the upper end of zone OSC as shownin FIG. 6( a) (or alternatively the lower end of zone OSC as shown inFIG. 6( b)). The arc length of zone OSC depends upon the specificgeometries of the workpiece being heat treated and the inductor designbeing used in a particular application; generally a non-limitinglimitation on arc length will be no greater than 150 millimeters; forexample, if workpiece 90 is a ring bearing race surface having an innerdiameter exceeding 1 meter, this initial heating oscillation zone arclength will be approximately 100 millimeters.

With AC current supplied to the inductors, the inductors oscillatebetween zone OSC start position and zone OSC stop position located atthe lower end of zone OSC as shown in FIG. 6( b). The initialoscillation zone heating provides a thermal barrier and reduced surfacecooling effect in zone OSC and continues until workpiece temperatures inzone OSC are sufficient to form homogeneous austenite within therequired hardening depth in this initial oscillatory zone OSC. Noquenchant is ejected from quench apparatus 14 a and 14 b during thisinitial oscillatory heating step. Consequently if the quench apparatusare mounted and moved separately from the inductors, they may remainstationary during the oscillatory heating step as opposed to moving withthe inductors as shown in FIG. 6( a) and FIG. 6( b).

At the end of the initial oscillatory zone OSC heating, inductors 12 aand 12 b separate and move in opposite directions through an arc lessthan a complete semicircle. For this example, as illustrated in FIG. 6(c) inductor 12 a (and associated quench apparatus 14 a) move through aclockwise (CW) arc to heat treat surface depths through points B1, B2and B3, while inductor 12 b (associated quench apparatus 14 b) movethrough a counterclockwise (CCW) arc to heat treat surface depthsthrough points A1, A2 and A3 typically at a constant (steady state) scan(speed) rate.

After inductors 12 a and 12 b separate a minimum distance at whichquenchant spray from the non-associated spray apparatus interferes withworkpiece heating of the non-associated inductor by impinging onworkpiece's regions being heated by the non-associated inductor, whichdistance is designated as the “spray interference distance,” sprayapparatus 14 a and 14 b are activated to release quenchant onto theheated workpiece regions as diagrammatically illustrated byrepresentative quench streams 14 a′ and 14 b′ in FIG. 6( d).

In the induction heat treatment process of the present invention, at theend of the steady state heat treatment process, inductors 12 a and 12 bapproach each other as shown in FIG. 7( a) less than 180 degreesopposite where the heat treatment process began. Both inductors 12 a and12 b continue the steady state heat treatment process until theside-by-side distance, d₂, between the inductors is as close aspermissible based on the inductor's physical configuration (includingtooling, mounting and support structure) as shown in FIG. 7( b). As onealternative final heat treatment process step, after inductor 12 acompletes heat treatment over and around surface region B4, aspositioned in FIG. 7( b), current to inductor 12 a is terminated andassociated quench apparatus 14 a is shut off (no spray). Inactiveinductor 12 a and inactive quench apparatus 14 a now move in thecounterclockwise direction while active inductor 12 b and associatedactive quench apparatus 14 b continue to move in the counterclockwisedirection from surface region A4 to surface region B4 as shown in FIG.7( c), preferably at: an increasing end of heat treatment scan speedgreater than the steady state scan rate; an end of heat treatment powermagnitude greater than the steady state power magnitude; and an end ofheat treatment frequency greater than the steady state frequency asfurther described below relative to FIG. 9( b) through FIG. 9( e). Theregion between surface regions A4 and B4 that inductor 12 b scans overto heat treat is referred to the “extended end scan region.”Alternatively inactive inductor 12 a and inactive quench apparatus 14 acan be removed from the heat treatment circular tracking path to allowmovement of inductor 12 b and quench apparatus 14 b through the extendedend scan region. After inductor 12 b completes its heat treatment in theextended end scan region that terminates over and around surface regionB4, its associated quench apparatus 14 b repositions as necessary tospray quench over and around surface regions A4-B3 as shown in FIG. 7(d), with the spray surface regions referred to as the “extended endspray region.” As another alternative final heat treatment process step,after inductors 12 a and 12 b complete heat treatment as positioned inFIG. 7( c), current to inductor 12 b is also terminated, and with quenchapparatus 14 a shutoff, inactive inductors 12 a and 12 b and activequench apparatus 14 b continue to counterclockwise to the position shownin FIG. 7( e) so that quench apparatus 14 b completes quench of surfaceregion B3. Alternatively inactive inductor 12 a and inactive quenchapparatus 14 a can be removed from the heat treatment circular trackingpath to allow inactive inductor 12 b and active quench apparatus 14 b tocontinue to move counterclockwise to the position shown in FIG. 7( e).The two above alternative examples for the end of (or final) heattreatment process step can be summarized as follows for the firstalternative example:

Surface Quench Inductor region Speed Power Frequency spray FIG. 12aB1-B3 SS SS SS ON 12b A1-A3 SS SS SS ON 12a B3-B4 SS SS SS ON 7(a)-7(b)12b A3-A4 SS SS SS ON 7(a)-7(b) 12a B4-B3 Inactive 0 0 OFF 7(b)-7(c) 12bA4-B4 >SS  >SS  >SS  ON 7(b)-7(c) 12a NA Inactive 0 0 OFF 7(c)-7(d) 12bA4-B3* Inactive 0 0 ON 7(c)-7(d) *Quench only by spray redirection.

and for the second alternative example:

Surface Scan Quench Inductor region Speed Power Frequency spray FIG. 12aB1-B3 SS SS SS ON 12b A1-A3 SS SS SS ON 12a B3-B4 SS SS SS ON 7(a)-7(b)12b A3-A4 SS SS SS ON 7(a)-7(b) 12a B4-B3 Inactive 0 0 OFF 7(b)-7(c) 12bA4-B4 >SS  >SS  >SS  ON 7(b)-7(c) 12a NA Inactive 0 0 OFF 7(e) 12bB4-B3** >SS  0 0 ON 7(e) **Final quench over surface regions B4-B3.

where “NA” indicates no surface heating or quench, and “SS” indicatessteady state scan speed, power magnitude or frequency.

In the induction heat treatment process of the present invention asdescribed above, a simultaneous “power-frequency” control scheme can beapplied that achieves the required thermal conditions of the heattreated regions. The initial and final heating process steps describedabove are, preferably, but not by way of limitation, performed withsimultaneous power-frequency control steps. FIG. 9( a) through FIG. 9(e) illustrate one preferred example of simultaneous variation of powerand frequency at different process stages. As discussed above, duringthe initial step of heating, oscillation of the pair of inductors 12 aand 12 b takes place (FIG. 6( a) and FIG. 6( b)). Lower frequency andlower power (than nominal heat treatment frequency and power, f_(nom)and P_(nom)) are supplied to the inductors during the pre-heatoscillation heating stage, for example, as shown in FIG. 9( b) throughFIG. 9( e) during the oscillation time period when both inductors areadjacent to the surface region in the oscillation region defined bysurface points “A1-A0-B0-B1” in these figures compared to a nominalsteady state heat treatment stage frequency (f_(nom)) and power(P_(nom)) during the time period when the inductors separate within theoscillatory zone and travel through surface regions “A1 to A3” and “B1to B3”. Since induced eddy current penetration is inversely proportionalto frequency, the initial oscillatory pre-heat stage provides requiredinitial thermal conditions (deep surface and low level heating) of theworkpiece region that will be initially heated. The initial thermalconditions can be selected to compensate for metal workpiece coolingduring the initial process delay in release of quenchant as describedabove when the side-by-side inductors are separating from each other.

Upon completion of an oscillating stage, the inductors start travelingin opposite circumferential directions and the heat treating (heatingand quenching) cycle continues according to the nominal steady stateconditions as shown in FIG. 6( e) and FIG. 9( b) through FIG. 9( e)during the time period when the inductors travel separated from eachother within the oscillatory zone, and travel through surface regions“A1 to A3” and “B1 to B3.” During the steady state heating stage, theapplied frequency and power densities of each inductor 12 a and 12 b areconstant, and the steady state frequency and power magnitude is greaterthan the corresponding frequency and power magnitude in the initialoscillating stage.

In contrast to the initial heating stage, in the final heating stage,power and frequency supplied to each inductor 12 a and/or 12 b increasesto provide sufficient thermal conditions at the end of heating byheating regions, which were not yet completely heated, according to theoptional end of heat treatment process that is utilized. Preferablysimultaneous variation of power and frequency at the initial and finalheating stages is performed in combination with the initial and finalheating stages described above. In one alternative final heating stage(FIG. 7( a), FIG. 7( b), FIG. 7( c) and FIG. 7( e)), one of theinductors is inactive (inductor 12 a in the example) and the otherinductor (inductor 12 b in the example) continues its movement andheating with frequency greater than steady state frequency; powermagnitude greater than steady state power, and scan rate greater thansteady state scan rate to maintain sufficient surface temperature forhardening areas that are quenched by quenchant from the quench apparatusas described above.

The above frequency-power control schemes may be accomplished with acomputer processor controlling the output of the power supplies to theinductors and electromechanical apparatus for coordinated movement ofthe inductors and quench apparatus.

Movement of the inductors and quench apparatus in one of the aboveexamples of the present invention, relative to the heating profiles inFIG. 9( a) through FIG. 9( e) is summarized in the following table.

Heat treatment stage Inductors Quench Frequency Power Oscillatory startSide-by-side No quench. Less than steady Less than steady zone pre-heatoscillatory state heat state power movement in treatment magnitude.start zone. frequency. Steady state heat Separation in the Quench startSteady state heat Steady state heat treatment from oscillatory startafter distance treatment treatment power start position to zone andtravel between frequency. magnitude. beginning of end around opposingseparating heat treatment circumferential inductors position. surface tothe exceeds spray finish (end) zone interference when inductorsdistance. are approximately side-by-side. Finish (end) zone Movement ofQuench control Generally higher Generally higher heat treatmentinductors based on inactive than steady state than steady stateaccording to and active heat treatment heat treatment selected optionalinductors frequency based power magnitude end of heat movement onselected with optional treatment through the optional end of correlationof process. extended end heat treatment scan speed spray region.process. control to refine heat treatment in finish zone.

In an alternative end of heat treatment process, inductors 12 a and 12 bapproach each other as shown in FIG. 10( a) less than 180 degreesopposite where the heat treatment process began. When the side-by-sidedistance, d₂, between the inductors is as close as permissible based onthe inductor's physical configuration (including tooling, mounting andsupport structure) as shown in FIG. 10( b), one of the two inductors,for example 12 a, is withdrawn from its heat treatment circular trackingpath, and the remaining inductor—inductor 12 b in this example—continuesto move in the counterclockwise direction to the position adjacent tothe circumferential surface that inductor 12 a was adjacent to before itwas withdrawn (FIG. 10( d) and FIG. 10( e)) to complete the end heattreatment process so that, in this alternative example, the entirecircumferential region of the outer circumferential surface of theworkpiece is uniformly metallurgically hardened. Both quench apparatus14 a and 14 b continue to direct quenchant spray to impinge upon theregion of the workpiece heated by inductor 12 a in the end treatmentprocess. Depending upon the relative mountings of the inductors and thespray apparatus, the directions of quenchant spray may be redirected byrotation of the spray apparatus as illustrated in FIG. 10( b) throughFIG. 10( e) to provide a more optimum quench impingement angle.

When inductor 12 b completes the workpiece heating process as shown inFIG. 10( d), inductor 12 b is withdrawn (removed) from the circulartracking path, which is in close proximity to the heated surface ofannular workpiece 90 as shown in FIG. 10( d). Quench apparatus 14 a and14 b provide quenching of the remaining heated area as shown in FIG. 10(e). At the very end of the quench cycle, quenchant spray may cease fromone of the quench apparatus (for example, quench apparatus 14 a) andquench block 14 b finishes the quenching process as shown in FIG. 10(e). Alternatively, depending upon the geometry of the workpiece, anadditional quench apparatus 14 c, might be applied at the final heatingposition to complement quenchant flow provided by quench apparatus 14 aand 14 b as shown in FIG. 10( f). Additional quench apparatus 14 c maybe optionally utilized in any other alternative end of heat treatmentprocess disclosed above.

FIG. 11 illustrates one example of an induction heating apparatus 30that can be used to perform some examples of the induction heattreatment process of the present invention. For convenience, and not byway of limitation of the invention, in FIG. 11 a three-dimensional X, Yand Z orthogonal coordinate system is designated to describe relativespatial relationships between components of the apparatus inthree-dimensional space. In FIG. 11, workpiece support assemblycomprises central support beam 32; extended arm support beams 34 a and34 b, joining arm support beams 36 a and 36 b, and workpiece retentionelements 38 a, 38 b and 38 c. Workpiece retention elements 38 a, 38 band 38 c are at least slidably mounted on central support beam 32;joining arm support beam 36 a; and joining support arm beam 36 b,respectively, to provide a three-point workpiece retention system. InFIG. 11, workpiece retention elements 38 a, 38 b and 38 c are shownpressing against the outer circumferential surface 88 a′ of annularworkpiece 88 for heat treatment of inner circumferential surface 88 a(and/or upper axial surface) of the workpiece. For heat treatment of theouter circumferential surface 88 a′, the retention elements would bepositioned to press against the inner circumferential surface 88 a ofthe workpiece by sliding each workpiece retention element on itsrespective beam so that all workpiece retention elements press againstthe inner circumferential surface. If inner and outer circumferentialsurfaces are heat treated at the same time, suitable means can beprovided to hold the workpiece in place without interference on eitherthe inner and outer circumferential surfaces by the workpiece retentionelements. For example the workpiece may be fixtured to a supportstructure that is secured by the workpiece retention elements asdescribed above. Further seating of the workpiece in the supportstructure is not restricted to having the workpiece oriented parallel toan X-Y plane; the workpiece may be otherwise oriented, for example, byaltering the height (Z-direction) of one or more of the workpieceretention elements. The workpiece support system of the presentinvention allows heat treatment of large annular workpieces with varyingdiameters with one apparatus 30.

In summary, if the annular workpiece 88 is a bearing race, the bearingrace support assembly as shown in FIG. 11, has a pair of extended armsupport beams 34 a and 34 b that extend at an acute angle at one oftheir ends from opposing sides and along the longitudinal length of thecentral support beam 32. The pair of joining arm support beams 36 a and36 b are connected between the extended ends of the pair of extended armsupport beams and opposing sides along the longitudinal length of thecentral support beam so that the joining arm support beam and extendedarm support beams form a “V” shaped frame on each side of thelongitudinal length of the central support beam. A separate workpieceretention element 38 a on the central support beam and each of the twojoining arm supports (retention elements 38 b and 380 can slide alongeach of these structures so that they can engage either the outer orinner circumferential surface of the bearing race or a fixture uponwhich the bearing race is seated.

Inductor assembly support and movement apparatus includes Y-axis(horizontally) oriented cross rail 42 and X-axis (horizontally) orientedextension rails 44 a and 44 b (partially shown) located at opposing endsof cross rail 42 that can extend to at least the diameter of the largestworkpiece that can be accommodated on the workpiece support assembly.The inductor assembly support and movement apparatus utilizes one ormore suitable drives 44 a and 44 b to move cross rail 42 along extensionrails 44 a and 44 b so that inductor assemblies 50 a and 50 b can movein the plus or minus X-direction over and around the workpiece.

Referring to FIG. 12, which is a detail view of inductor assemblies 50 aand 50 b, first inductor 12 a is connected to electrical component 52 a,which may comprise a load matching transformer and/or other electricalcontrol circuitry. Electrical component 52 a is connected to a suitablealternating current power source (not shown in the figures) that can beremotely located. Electrical component 52 a can be pivotally connectedto vertical column support 60 a by pivot element 61 a that allowselectrical component 52 a (and connected inductor 12 a) to rotate aboutaxis X1. Vertical support column 60 a can raise and lower first inductor12 a in the Z-direction by suitable driver 64 a while vertical supportcolumn 60 a is slidably attached to cross rail 42, which allows thevertical column (and indirectly connected first inductor 12 a) to movein the plus or minus Y-direction via driver 63 a. Linear actuator 62 ais attached between the common support for pivot element 61 a and thetop of electrical component 52 a with horizontal offset from the pivotpoint connection, which allows linear actuator 62 a to rotate firstinductor 12 a in a Y-Z plane. A suitable driver is provided to rotatefirst inductor 12 a in an X-Y plane.

Second inductor assembly 50 b is similarly to, but independent from,first inductor assembly 50 a. Second inductor 12 b is connected toelectrical component 52 b, which may comprise a load matchingtransformer and/or other electrical control circuitry. Electricalcomponent 52 b is connected to a suitable alternating current powersource (not shown in the figure) that can be remotely located. A commonor different power source may be used for each inductor depending upon aparticular application. Electrical component 52 b can be pivotallyconnected to vertical support column 60 b by pivot element 61 b thatallows electrical component 52 b (and connected inductor 12 b) to rotateabout axis X2. Vertical support column 60 b can raise and lower secondinductor 12 b in the Z-direction by suitable driver 64 b while verticalsupport column 60 b is slidably attached to cross rail 42, which allowsthe vertical column (and indirectly connected second inductor 12 b) tomove in the plus or minus Y-direction via driver 63 a. Linear actuator62 b is attached between the common support for pivot element 61 b andthe top of electrical component 52 b with horizontal offset from thepivot point connection, which allows linear actuator 62 b to rotatesecond inductor 12 b in a Y-Z plane. A suitable driver is provided torotate second inductor 12 b in an X-Y plane.

Extension of linear actuator 62 b and refraction of linear actuator 62 awill cause both first and second inductors to rotate clockwise off ofvertical in a Y-Z plane. With the positioning system described above thefirst and second inductors can move with multiple degrees of freedom.With suitable programming a process controller can be used to controlall of the drivers associated with the above actuators and drivemechanisms.

Quench apparatus is not shown in FIG. 11 and FIG. 12, but can besuitably affixed to the inductors tooling or support structure, or canbe mounted independently and adjacent to the inductors, and can also bepivotally mounted relative to the inductors if required for a particularapplication.

The apparatus in FIG. 11 and FIG. 12 can be applied to one example ofthe heat treatment process of the present invention. For clarity heattreatment of only the outer circumferential surface of annular workpiece88 will be described although simultaneous heat treatment of both theinner and outer circumferential surfaces can be performed. Assuming thatworkpiece 88 lies in an X-Y plan below the initial height (Z-direction)of inductors 12 a and 12 b. Drivers 44 a and 44 b are activated to movecross rail 42 (and inductors 12 a and 12 b) towards the outercircumferential surface of workpiece 88, and drivers 64 a and 64 b(providing a means for linearly moving each of the pair of inductorsindependently in a plane parallel to the central axis of the annularworkpiece) are activated to lower (Z-direction) the inductors to theinitial location of the heat treatment circular tracking path withside-by-side inductors adjacent to the outer circumferential surface ofthe workpiece. Drivers 44 a and 44 b and 63 a and 63 b are coordinatelyactivated to produce an oscillatory X-Y directions movement (providing ameans for linearly moving each of the pair of inductors in a planeperpendicular to the central axis of the bearing race in the first andsecond orthogonal directions of the perpendicular plane) whilerotational drivers for inductors 12 a and 12 b are utilized tocoordinately rotate inductors 12 a and 12 b independently about axis Z1_(R) and Z2 _(R) to perform a pre-heat oscillatory heat treatmentprocess step as disclosed above. After completion of the pre-heatoscillatory step, drivers 44 a and 44 b and 63 a and 63 b arecoordinately activated, while rotational drivers for inductors 12 a and12 b are utilized to coordinately rotate inductors 12 a and 12 bindependently about axis Z1 _(R) and Z2 _(R) to move inductors 12 a and12 b in opposite directions around the outer circumferential surface ofthe workpiece in a steady state heat treatment process step as disclosedabove until inductors 12 a and 12 b reach the final (end of heattreatment) heating zone. After completion of the steady state heattreatment process step, drivers 44 a and 44 b and 63 a and 63 b arecoordinately activated, while rotational drivers for inductors 12 a and12 b are utilized to coordinately rotate inductors 12 a and 12 bindependently about axis Z1 _(R) and Z2 _(R) to move inductors 12 a and12 b as disclosed in one of the end heat treatment process steps asdisclosed above. In this example of the invention spray apparatus 14 aand 14 b, which are respectively associated with inductors 12 a and 12 bare mounted and moved coordinately with their associated inductorsduring execution of the pre-heat oscillatory step; the steady state heattreatment process step and the end heat treatment process step.

The apparatus shown in FIG. 11 and FIG. 12 may also be utilized for gearteeth hardening, and is particularly advantageous for hardening of gearswith spiral teeth. In existing applications the gear must be rotated toaccommodate a fixed inductor whereas in the apparatus shown the gear canremain stationary, and the same arrangement can also be used for gearswith straight gear teeth.

While the term “circular” is used in the examples, the term as usedherein also includes elliptically shaped workpieces. Although the aboveexamples of the invention utilize a single pair of inductors, any numberof inductor pairs could be used according to the process described aboveto increase production rates, with the appropriate decrease in theapproximately 180 degrees arc of a complete circular surface heattreated by each pair of inductors. For example, if two inductor pairsare utilized, then each pair would heat treat an approximately 90degrees arc of the complete circular surface. Although the aboveexamples of the invention illustrate the process for outer(circumferential or peripheral) diameter heat treatment of the annularworkpiece, the process can also be applied to inner diameter heattreatment of the annular workpiece, as well as the width (side or axialsurfaces) of the annulus. Depending upon the application, heating can beapplied to the outer or inner diameters of the ring or both. In otherapplications, the side surface of the ring alone, or in addition toouter and/or inner diameters of the ring may be heat treated by theprocess of the present invention.

The present invention has been described in terms of preferred examplesand embodiments. Equivalents, alternatives and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

The invention claimed is:
 1. A method of electric induction heattreatment of at least one circular surface of an annular workpiece, themethod comprising the steps of: positioning side-by-side a first and asecond inductor at an initial position adjacent to the at least onecircular surface, the initial position located within an oscillatory arczone of the at least one circular surface, the oscillatory arc zonehaving a first and a second arc boundary; supplying an oscillatory zonealternating current to the first and second inductors whilecircumferentially moving side-by-side the first and second inductorsrepeatedly between the first and second arc boundaries for a pre-heatperiod of time while adjacent to the at least one circular surface;supplying a steady state heat treatment power having a steady statepower magnitude and frequency to the first and second inductors whileseparating the first and the second inductors in the oscillatory arczone by moving the first inductor in a first circumferential directionadjacent to the at least one circular surface to a first inductor endsteady state heat treatment position less than 180 degrees opposite theinitial position at a steady state scan rate, and by moving the secondinductor in a second circumferential direction adjacent to the at leastone circular surface to a second inductor end steady state heattreatment position at the steady state scan rate, the secondcircumferential direction opposite to the first circumferentialdirection; directing a first quenchant spray from a first quenchapparatus to impinge on a first inductor heated region of the at leastone circular surface heated by the first inductor as the first inductormoves in the first circumferential direction to the first inductor endsteady state heat treatment position after the first inductor separatesfrom the second inductor by a spray interference distance, and directinga second quenchant spray from a second quench apparatus to impinge on asecond inductor heated region of the at least one circular surfaceheated by the second inductor as the second inductor moves in the secondcircumferential direction to the second inductor end steady state heattreatment position after the second inductor separates from the firstinductor by the spray interference distance; removing the steady stateheat treatment power from the first inductor and terminating the firstquenchant spray after the first inductor completes heat treatment at thefirst inductor end steady state heat treatment position; moving thesecond inductor in the second circumferential direction after the secondinductor completes heat treatment at the second inductor end steadystate heat treatment position to the end of an extended end scan regionto heat treat the extended end scan region at an end of heat treatmentscan rate faster than the steady state scan rate and at an end of heattreatment power magnitude and frequency; and directing the secondquenchant spray to impinge on the extended end spray region byalternatively repositioning the second quench apparatus while the secondinductor is at the end of the extended end scan region or moving thesecond quench apparatus through the extend end spray region.
 2. Themethod of claim 1 wherein the step of supplying the oscillatory zonealternating current to the first and second inductors is initiated whenthe first and second inductors are located side-by-side at the first orsecond arc boundary.
 3. The method of claim 1 wherein the separation ofthe first and second inductors in the oscillatory arc zone is initiatedin the center of the oscillatory arc zone.
 4. The method of claim 1wherein the end of heat treatment frequency is greater than the steadystate frequency, and the end of heat treatment power magnitude isgreater than the steady state power magnitude.
 5. The method of claim 1wherein the step of supplying the oscillatory zone alternating currentfurther comprises supplying the oscillatory zone alternating current ata pre-heat frequency less than the steady state frequency, and at apre-heat power magnitude less than the steady state power magnitude. 6.The method of claim 5 wherein the end of heat treatment frequency isgreater than the steady state frequency, and the end of heat treatmentpower magnitude is greater than the steady state power magnitude.
 7. Amethod of electric induction heat treatment of at least one circularsurface of an annular workpiece, the method comprising the steps of:positioning side-by-side a first and a second inductor at an initialposition adjacent to the at least one circular surface, the initialposition located within an oscillatory arc zone of the at least onecircular surface, the oscillatory arc zone having a first and a secondarc boundary; supplying an oscillatory zone alternating current to thefirst and second inductors while circumferentially moving side-by-sidethe first and second inductors repeatedly between the first and secondarc boundaries for a pre-heat period of time while adjacent to the atleast one circular surface; supplying a steady state heat treatmentpower having a steady state magnitude and frequency to the first andsecond inductors while separating the first and the second inductors inthe oscillatory arc zone by moving the first inductor in a firstcircumferential direction adjacent to the at least one circular surfaceto a first inductor end of steady state heat treatment position lessthan 180 degrees opposite the initial position at a steady state scanrate, and by moving the second inductor in a second circumferentialdirection adjacent to the at least one circular surface to a secondinductor end steady state heat treatment position at the steady statescan rate, the second circumferential direction opposite to the firstcircumferential direction; directing a first quenchant spray from afirst quench apparatus to impinge on a first inductor heated region ofthe at least one circular surface heated by the first inductor as thefirst inductor moves in the first circumferential direction to the firstinductor end steady state heat treatment position after the firstinductor separates from the second inductor by a spray interferencedistance, and directing a second quenchant spray from a second quenchapparatus to impinge on a second inductor heated region of the at leastone circular surface heated by the second inductor as the secondinductor moves in the second circumferential direction to the secondinductor end steady state heat treatment position after the secondinductor separates from the first inductor by the spray interferencedistance; removing the steady state heat treatment power from the firstinductor and terminating the first quenchant spray after the firstinductor completes heat treatment at the first inductor end steady stateheat treatment position; moving the second inductor in the secondcircumferential direction after the second inductor completes heattreatment at the second inductor end steady state heat treatmentposition to the end of an extended end scan region to heat treat theextended end scan region at an end of heat treatment scan rate fasterthan the steady state scan rate and at an end of heat treatment powermagnitude and frequency; moving the second inductor in the secondcircumferential direction after the second inductor completes heattreatment to the end of the extended end scan region to a distancebeyond the end of the extended end spray region so that the secondquenchant spray impinges on the extended end spray region.
 8. The methodof claim 7 wherein the step of supplying the oscillatory zonealternating current to the first and second inductors is initiated whenthe first and second inductors are located side-by-side at the first orsecond arc boundary.
 9. The method of claim 7 wherein the separation ofthe first and second inductors in the oscillatory arc zone is initiatedin the center of the oscillatory arc zone.
 10. The method of claim 7wherein the end of heat treatment frequency is greater than the steadystate frequency, and the end of heat treatment power magnitude isgreater than the steady state power magnitude.
 11. The method of claim 7wherein the step of supplying the oscillatory zone alternating currentfurther comprises supplying the oscillatory zone alternating current ata pre-heat frequency less than the steady state frequency, and at apre-heat power magnitude less than the steady state power magnitude. 12.The method of claim 11 wherein the end of heat treatment frequency isgreater than the steady state frequency, and at the end of heattreatment power magnitude greater than the steady state power magnitude.13. A method of electric induction heat treatment of at least onebearing race having an inner diameter of at least one meter, the methodcomprising the steps of: positioning side-by-side a first and a secondinductor at an initial position adjacent to the at least one bearingrace, the initial position located within an oscillatory arc zone of theat least one bearing race, the oscillatory arc zone having a first and asecond arc boundary; supplying an oscillatory zone alternating currentto the first and second inductors while circumferentially movingside-by-side the first and second inductors repeatedly between the firstand second arc boundaries for a pre-heat period of time while adjacentto the at least one bearing race; supplying a steady state heattreatment power having a steady state power magnitude and frequency tothe first and second inductors while separating the first and the secondinductors in the oscillatory arc zone by moving the first inductor in afirst circumferential direction adjacent to the at least one bearingrace to a first inductor end steady state heat treatment position lessthan 180 degrees opposite the initial position at a steady state scanrate, and by moving the second inductor in a second circumferentialdirection adjacent to the at least one bearing race to a second inductorend steady state heat treatment position at the steady state scan rate,the second circumferential direction opposite to the firstcircumferential direction; directing a first quenchant spray from afirst quench apparatus to impinge on a first inductor heated region ofthe at least one bearing race heated by the first inductor as the firstinductor moves in the first circumferential direction to the firstinductor end steady state heat treatment position after the firstinductor separates from the second inductor by a spray interferencedistance, and directing a second quenchant spray from a second quenchapparatus to impinge on a second inductor heated region of the at leastone bearing race heated by the second inductor as the second inductormoves in the second circumferential direction to the second inductor endsteady state heat treatment position after the second inductor separatesfrom the first inductor by the spray interference distance; removing thesteady state heat treatment power from the first inductor andterminating the first quenchant spray after the first inductor completesheat treatment at the first inductor end steady state heat treatmentposition; moving the second inductor in the second circumferentialdirection after the second inductor completes heat treatment at thesecond inductor end steady state heat treatment position to the end ofan extended end scan region to heat treat the extended end scan regionat an end of heat treatment scan rate faster than the steady state scanrate and at an end of heat treatment power magnitude and frequency; anddirecting the second quenchant spray to impinge on an extended quenchregion by repositioning the second quench apparatus while the secondinductor is at the end of the extended end scan region.
 14. The methodof claim 13 wherein the step of supplying the oscillatory zonealternating current to the first and second inductors is initiated whenthe first and second inductors are located side-by-side at the first orsecond arc boundary.
 15. The method of claim 13 wherein the separationof the first and second inductors in the oscillatory arc zone occurs inthe center of the oscillatory arc zone.
 16. The method of claim 13wherein the end of heat treatment frequency is greater than the steadystate frequency, and the end of heat treatment power magnitude isgreater than the steady state power magnitude.
 17. The method of claim13 wherein the step of supplying the oscillatory zone alternatingcurrent further comprises supplying the oscillatory zone alternatingcurrent at a pre-heat frequency less than the steady state frequency,and at a pre-heat power magnitude less than the steady state powermagnitude.
 18. The method of claim 17 wherein the end of heat treatmentfrequency is supplied at a frequency greater than the steady statefrequency, and the end of heat treatment power magnitude is greater thanthe steady state power magnitude.
 19. A method of electric inductionheat treatment of at least one bearing race having an inner diameter ofat least one meter, the method comprising the steps of: positioningside-by-side a first and a second inductor at an initial positionadjacent to the at least one bearing race, the initial position locatedwithin an oscillatory arc zone of the at least one bearing race, theoscillatory arc zone having a first and a second arc boundary; supplyingan oscillatory zone alternating current to the first and secondinductors while circumferentially moving side-by-side the first andsecond inductors repeatedly between the first and second arc boundariesfor a pre-heat period of time while adjacent to the at least one bearingrace; supplying a steady state heat treatment power having a steadystate magnitude and frequency to the first and second inductors whileseparating the first and the second inductors in the oscillatory arczone by moving the first inductor in a first circumferential directionadjacent to the at least one bearing race to a first inductor end steadystate heat treatment position less than 180 degrees opposite the initialposition at a steady state scan rate, and by moving the second inductorin a second circumferential direction adjacent to the at least onebearing race to a second inductor end steady state heat treatmentposition at the steady state scan rate, the second circumferentialdirection opposite to the first circumferential direction; directing afirst quenchant spray from a first quench apparatus to impinge on afirst inductor heated region of the at least one bearing race heated bythe first inductor as the first inductor moves in the firstcircumferential direction to the first inductor end steady state heattreatment position after the first inductor separates from the secondinductor by a spray interference distance, and directing a secondquenchant spray from a second quench apparatus to impinge on a secondinductor heated region of the at least one bearing race heated by thesecond inductor as the second inductor moves in the secondcircumferential direction to the second inductor end steady state heattreatment position after the second inductor separates from the firstinductor by the spray interference distance; removing the steady stateheat treatment power from the first inductor and terminating the firstquenchant spray after the first inductor completes heat treatment at thefirst inductor end steady state heat treatment position; moving thesecond inductor in the second circumferential direction after the secondinductor completes heat treatment at the second inductor end steadystate heat treatment position to the end of an extended end scan regionto heat treat the extended end scan region at an end of heat treatmentscan rate faster than the steady state scan rate and at an end of heattreatment power magnitude and frequency; and moving the second inductorin the second circumferential direction after the second inductorcompletes heat treatment to the end of the extended end scan region to adistance beyond the end of the extended quench region so that the secondquenchant spray impinges on an extended end spray region.
 20. The methodof claim 19 wherein the step of supplying the oscillatory zonealternating current to the first and second inductors is initiated whenthe first and second inductors are located side-by-side at the first orsecond arc boundary.
 21. The method of claim 19 wherein the separationof the first and second inductors in the oscillatory arc zone occurs inthe center of the oscillatory arc zone.
 22. The method of claim 19wherein the end of heat treatment frequency is greater than the steadystate heat treatment frequency, and the end of heat treatment powermagnitude is greater than the steady state heat treatment powermagnitude.
 23. The method of claim 19 wherein the step of supplying theoscillatory zone alternating current further comprises supplying theoscillatory zone alternating current at a pre-heat frequency less thanthe steady state heat treatment frequency, and at a pre-heat powermagnitude less than the steady state power magnitude.
 24. The method ofclaim 23 wherein the end of heat treatment frequency is supplied at afrequency is greater than the steady state heat treatment frequency, andat the end of heat treatment power magnitude greater than the steadystate heat treatment power magnitude.